fix: display new password after admin password reset

- show-admin-credentials.js --reset now displays the generated password
  instead of just saying "[NEWLY RESET - stored in database]"
- Also sets must_change_password flag to force password change on login
- Updated DEPLOYMENT_GUIDE.md and SIMPLE_SETUP.md to clarify that the
  new password is displayed in console output after reset

SIMPLE_SETUP.md

@@ -451,6 +451,8 @@ cd /opt/picpeak/app/backend
 sudo -u picpeak node scripts/reset-admin-password.js
 ```
 
+> **Note:** The new password will be displayed in the console output and saved to `ADMIN_PASSWORD_RESET.txt`. Save it immediately!
+
 ### Getting Help
 
 1. **Check logs:**